Thin Film Encapsulation for LCP-Based Flexible Bioelectronic Implants: Comparison of Different Coating Materials Using Test Methodologies for Life-Time Estimation

Abstract

Liquid crystal polymer (LCP) has gained wide interest in the electronics industry largely due to its flexibility, stable insulation and dielectric properties and chip integration capabilities. Recently, LCP has also been investigated as a biocompatible substrate for the fabrication of multielectrode arrays. Realizing a fully implantable LCP-based bioelectronic device, however, still necessitates a low form factor packaging solution to protect the electronics in the body. In this work, we investigate two promising encapsulation coatings based on thin-film technology as the main packaging for LCP-based electronics. Specifically, a HfO₂-based nanolaminate ceramic (TFE1) deposited via atomic layer deposition (ALD), and a hybrid Parylene C-ALD multilayer stack (TFE2), both with a silicone finish, were investigated and compared to a reference LCP coating. T-peel, water-vapour transmission rate (WVTR) and long-term electrochemical impedance spectrometry (EIS) tests were performed to evaluate adhesion, barrier properties and overall encapsulation performance of the coatings. Both TFE materials showed stable impedance characteristics while submerged in 60 °C saline, with TFE1-silicone lasting more than 16 months under a continuous 14V DC bias (experiment is ongoing). The results presented in this work show that WVTR is not the main factor in determining lifetime, but the adhesion of the coating to the substrate materials plays a key role in maintaining a stable interface and thus longer lifetimes.

Keywords: Parylene-C (ParC); atomic layer deposition (ALD); flexible bioelectronics; liquid crystal polymer (LCP); long-term encapsulation; thin-film encapsulation (TFE).

Figure 1

Generic sketch of a wirelessly powered fully implantable active device based on a polymer substrate: electronic components in the form of capacitor and ASIC constituting the functional circuit; metal electrodes for delivering or receiving signals to/from the tissue; coil for wireless communication and power transmission. All implemented into a flexible biocompatible polymer covered with a thin film (TFE) encapsulation layer.

Figure 2

Schematic cross-section illustration of the tested samples: LCP-TFE1/2 samples used for pre-screening test (a), LCP-LCP laminate (b) and LCP-TFE1/2 with silicone finish samples (c) used for sorption tests, LCP-LCP (d) and LCP-TFE1/2 (e) with silicone finish samples used for adhesion T-peel test. Adapted ASTM D1876 T-Peel test setup with the sample (f). Schematic representation of the IDC tested samples: LCP-Au-LCP (g) and LCP-Au-TFE1/2 with silicone finish (h). Exposed Au IDC test structure on LCP (i).

Figure 3

SEM images of bare LCP (a) and LCP coated with a 100 nm thick HfO₂-based ALD ML (TFE1) (b). Cross-sectional TEM images of 100 nm HfO₂-based ALD ML on LCP: (c) before soak; (d) after 2 months soak in PBS at 67 °C.

Figure 4

SEM images of LCP (a,c) and PDMS (b,d) surfaces after T-peel test before and after soaking.

Figure 5

Lifetime of different coatings on LCP substrates with Au IDCs in 60 °C PBS. End of a sample’s lifetime was indicated by a >10% deviation of the impedance magnitude at 0.1 Hz, compared to the original value at the beginning of the aging study. Red asterisks (*) represent failed samples at the corresponding month.